The present invention relates generally to a television lighting control system, and more particularly to an energy-efficient system of lighting the field of view of a television camera.
There are many applications when people who control a portion of land desire to keep other people from entering their portion of land. For example, a security force may be tasked with preventing unauthorized persons from crossing a strip of land bordering a prison, industrial plant, country, etc. For most such applications, indiscriminate life threatening devices such as vicious dogs and land mines may provide adequate security but they have consequences not always acceptable to society. Therefore, the controllers must trade-off the amount of money they will spend on acceptable security measures versus their risk if the money is not spent.
One of the most effective, but expensive, acceptable security measures is a human guard. To minimize the number of guards needed, television is often used so that one centrally located guard can monitor a number of remote sites. In this situation, each remote site has at least one television camera for transmitting a picture of a viewing area to a television monitor at the guard's location. If unauthorized activity is noted on the television monitor, the guard may take appropriate action.
A television camera has an optical system including a lens for receiving light from the viewing area and an active vacuum or solid-state element for converting impinging light to an electrical signal. Natural light during daylight hours is usually so bright as to saturate the active element. Therefore, the optical system further includes a light control means for controlling the ratio of light leaving to light entering the optical system from a maximum value to a minimum value. This means is usually an opaque contractile diaphram, or iris, that automatically closes ("stops down") or opens to quickly adjust the light impinging the active element to a proper intensity.
At night the cost of a television monitoring system increases because artificial lighting must be used to illuminate the camera field of view. Typical prior applications have lighting control independent of the television camera. In these applications, the light intensity must be sufficient to ensure a proper television picture under the worst possible lighting conditions, such as the middle of the night of a new moon. In other situations where there is more natural light, the automatic light control system of the television camera "stops down" to limit the sum of natural and artificial light at the active element to the proper value.
In this invention, applicant recognizes that it is more economical to turn down the artificial light with the lens nearly wide open then to stop down the lens with the artificial light on full power.